Either buy a code reader from Napa or Autozone or wherever for $50, or spend a couple grand on a real one from your local Snapon or OTC guy. There really isn't a lot of middle ground, unless you find an old one a pro is getting rid of. I am completly unfamiliar with PC based things. Proper ones are real money, I imagine there is something hacked together in a basement for less.

Your Miata won't give much data- the Asians were a bit parsimonious with their datastream at that time. May only flash codes. The Diesel may talk to a generic scannerbut I don't know. I avoid oil burners at my shop.

And I use the phrase "real one" advisedly. There is no substitute for the factory tool for a professional.